Fig. 4. An anterior-posterior Twist gradient induces ectopic expression of sim. Embryos were hybridized with a digoxigenin-labeled sim (A-F) or Sex-lethal (G-I) antisense RNA probe and are oriented with anterior to the left. (A,D) Wild-type embryos that lack (A) or express (D) a twist-bcd transgene. sim is normally expressed in two lines that straddle the presumptive mesoderm (A). Staining does not extend to the anterior pole. In contrast, the twist-bcd transgene induces ectopic expression of sim at the anterior pole (D). This staining is first detected in precellular embryos (not shown), and persists during cellularization (D) and gastrulation (not shown). (B,C) snail^–/snail^– mutant embryos that lack the twist-bcd transgene. There is a delay in the onset of expression, and staining is not detected in advanced cellularizing embryos (B). Sporadic and weak expression is first detected at the onset of gastrulation in ventral regions (C). B and C display lateral and ventral views, respectively. (E,F) snail^–/snail^– mutant embryos that express the twist-bcd transgene. There is strong activation of sim in anterior regions of cellularizing embryos (E and F display lateral and ventral reviews, respectively). Strong ectopic expression persists during germband elongation (not shown). (G-I) Sex-lethal expression in wild-type embryos and those expressing the twist-bcd transgene. Sex-lethal is ubiquitously expressed in female embryos (H) and is not expressed in male embryos (G). In embryos from females containing the twist-bcd transgene, Sex-lethal is repressed at the anterior end of the embryo (I) in the domain that coincides with ectopic Twist expression. All Sex-lethal expressing embryos exhibit this repression, which accounts for the daughterless phenotype exhibited by females containing the twist-bcd transgene.